Electronic or e-cigarettes have grown in popularity over the past several years, especially among teenagers. The U.S. Centers for Disease Control and Prevention found that e-cigarette use among American teens tripled from 2013 to 2014. A new study funded by the National Institutes of Health now shows teens who use e-cigarettes are more likely than those who don't to smoke tobacco.

Co-author of the study Adam Leventhal, an associate professor at the University of Southern California (USC), specializes in research on smoking cessation and the causes of addiction. Although e-cigarettes come in tobacco flavors, he observed that many of the flavors are those that teens like. “They come in attractive flavors like cotton candy, bubble gum. They’re very hi-tech and you know, they’re modern and cool,” he said.

E-cigarettes, e-cigs, e-hookas, which refer to the same product, are essentially nicotine delivery devices. They use a liquid solution containing varying levels of nicotine that is inhaled, a process often called “vaping.” E-cigarettes have helped some tobacco users wean themselves from nicotine, but teens seem to be attracted to the drug.

Studies show nicotine can affect a teenager's memory. It can increase tension, depression and anxiety. Some doctors are concerned that e-cigarettes set up the adolescent brain for more durable and stronger addiction pathways, not just for nicotine, but for substances such as cocaine, marijuana and other drugs.

The USC researchers surveyed 2,300 Los Angeles area high school students starting at the beginning of ninth grade when most students were about 14 years old. The study also compared tobacco use initiation among 222 students who had used e-cigarettes, but not combustible tobacco products.

“We compared teens who had used e-cigarettes to teens who hadn’t, and we looked to see whether there were differences between those two groups and the initiation of smoking of combustible tobacco products," Leventhal said. The students were surveyed again six months later and at the start of 10th grade.

“The teens who used e-cigarettes were more than four times more likely to start smoking than the teens who hadn’t used e-cigarettes," said Leventhal. He said the study doesn't show that e-cigarette use directly leads to smoking, but it increases the risk.

“If they’re inhaling nicotine and they enjoy the experience of inhaling nicotine via the e-cigarette," Leventhal said, "it’s possible that they might be more open to experimenting with smokeable tobacco products in the future.”

The study also raises issues about teen smoking. “While teen tobacco use has fallen in recent years, this study confirms that we should continue to vigilantly watch teen smoking patterns,” said Dr. Nora Volkow, director of the National Institute on Drug Abuse (NIDA) at NIH. “Parents and teens should recognize that although e-cigarettes might not have the same carcinogenic effects of regular cigarettes, they do carry a risk of addiction,” she said.

Watch Dr. Nora Volkow, Director of the National Institute of Drug Abuse talk about the study:

"Classically, it is recognized that e-cigarettes are likely to be much less toxic than combustible tobacco because you don't have the carcinogenic chemicals that you get when you are smoking a cigarette. But on the other hand, nicotine is an addictive substance, and by that nature it produces neuroplastic changes in the brain, and so our concern has been whether early exposure of adolescents to nicotine could prime their brain to the rewarding effects of other drugs including tobacco, regular or combustible tobacco. And so this study was addressing in a longitudinal way, that whether kids that get exposed to electronic cigarettes, and therefore exposed to nicotine, are then more likely to have increased risk for smoking the regular cigarettes and that's actually what they seem to have found."

The U.S. Food and Drug Administration says it plans to regulate e-cigarettes, along with cigars and other tobacco products that are not currently regulated in the United States. Some countries restrict the sale of certain types of e-cigarettes, but they are widely available on the Internet which makes enforcement difficult.

The study was published in the Journal of the American Medical Association.